1. Field of the Invention
The invention relates in general to a method and a device for generating a stable power control signal in a CD recorder, and more particularly to a method for controlling a power control signal of a laser diode such that the read power control voltage is automatically and dynamically adjusted as a feedback control signal varies, and a read level, an erase level and a write level of the power control signal are also automatically and dynamically adjusted as the read power control voltage varies.
2. Description of the Related Art
When a conventional CD-RW (CD-ReWritable) recorder is writing a disk, a power control signal for controlling a laser diode may be a read level V_bias, an erase level V_erase and a write level V_write. When the recorder wants to read a CD-RW, the level of the power control signal is the read level V_bias. When the recorder is erasing data on the CD-RW disk, the level of the power control signal is the erase level V_erase. When the recorder wants to write data on the CD-RW disk, the level of the power control signal is the write level V_write. The power control signal controls the laser diode to generate a laser beam to read, erase, or write the CD-RW disk.
The CD-RW recorder further has a monitor diode for generating a current iM according to the intensity of the laser beam generated by the laser diode. An OP amplifier processes the current iM and then generates a feedback control signal FPDO. FIG. 1 shows a waveform of a feedback control signal FPDO generated when the conventional CD-RW recorder is writing. In the time slot T1, the CD-RW recorder is reading the disk. In the time slot T2, the CD-RW recorder is erasing the data on the disk. In the time slot T3, the CD-RW recorder forms the land of the CD-RW disk. In the time slot T4, the CD-RW recorder forms a pit on the disk.
In general, the read level V_bias, the erase level V_erase and the write level V_write are generated by generating a read power control voltage VRDCO, a first write power control voltage VW1DCO and a second write power control voltage VW2DCO. The read level V_bias equals a sum of a reference voltage Vref and the read power control voltage VRDCO, the erase level V_erase equals a sum of the read level V_bias and the first write power control voltage VW1DCO, and the write level V_write equals a sum of the erase level V_erase and the second write power control voltage VW2DCO.
FIG. 2 is a block diagram showing a circuit for generating three power control voltages in a conventional CD recorder. A read DAC (Digital-to-Analog Converter, RDAC) 202 selects one of a plurality of stored digital values, converts the selected digital value into an analog signal A1, and outputs the analog signal A1 to a first low-pass filter (LPF) 204 in order to obtain the read power control voltage VRDCO. In the correction process before the product is shipped out, a most suitable digital value may be corrected as the output of the RDAC 202. On the other hand, the first write power control voltage VW1DCO and the second write power control voltage VW2DCO are generated as follows. First, the feedback control signal FPDO is inputted to a sample and hold circuit (S/H) 206, which generates an analog signal A2. On the other hand, a write target value digital-to-analog converter (WDAC) 208 selects one of the digital values and converts the selected digital value into an analog signal A3 for output. The analog signal A2 and the analog signal A3 are added together to obtain a sum, and then the sum is inputted to a second low-pass filter 210 to obtain a first power control voltage VW1DCO. An amplifier 212 with the gain G amplifies the first power control voltage VW1DCO by G times to obtain a second power control voltage VW2DCO.
The S/H 206 samples the feedback control signal FPDO according to a pulse signal E_Pulse of FIG. 1 during the erasing operation. The first power control voltage VW1DCO may be adjusted as the feedback control signal FPDO varies through a close loop control system composed of the S/H 206 and the second LPF 210.
However, because the circuit for generating the read power control voltage VRDCO is an open loop control system, the read power control voltage VRDCO will change as the temperature rises after a long term usage of the recorder. At this time, the read level V_bias, the first power control voltage VW1DCO and the second power control voltage VW2DCO will shift and errors occur, and the write quality is influenced.